Mesh moves back in Edit Mode
Any poses etc. are moving and/or deforming the mesh are doing these operations on top the basic mesh. When you go to Edit Mode, the mesh shows its original basic appearance, to put it in simply words. It's the same with modifiers for example, if you take a cube and put a Subdivision Surface modifier on it, the cube becomes rounded. But going into Edit Mode shows the original basic cube (unless you have set the option to show the modifier effect in Edit Mode as well). This is usually useful if you want to tweak or change something general which is easier to do on the basic low-poly cube mesh than on a high-poly mesh.